In gas processing at cryogenic temperatures, particularly with air separation processes, it is required to remove impurities from the feed stream. In the case of an air separation process, moisture, carbon dioxide and hydrocarbon components must be removed to minimum levels prior to the feed stream being processed to cryogenic temperatures. A conventional method used to carry out removal of contaminants from such streams is adsorption whereby the gaseous feed stream is contacted with adsorbents to transfer components from the gaseous feed stream to the adsorbent material. In the case of air separation, the adsorption process is conducted by passing the incoming feed stream of compressed air through a first adsorbent material which preferentially adsorbs water and thereafter through a second adsorption material which preferentially adsorbs carbon dioxide. Adsorbent material requirements are somewhat reduced in a multiadsorbent system due to the removal of water, the more strongly sorbed component prior to removal of less strongly sorbed components.
For all adsorber systems, there are design and operational considerations common to all. It is usually necessary to have a minimum of two adsorbers. One adsorber is in production and the other available for regeneration, usually using dry gas. In the case of air separation, the quantity of the first sorbent material is typically on the order of between about 50% and 70% of that quantity of second sorbent material depending on feed gas temperature. For air separation, the first sorbent is typically alumina spheres and the second sorbent material is molecular sieve. Typically in existing adsorber systems, the alumina spherical diameters are about 3 to 6 millimeters in diameter, and the molecular sieves are between about 1 and 3 millimeters in diameter. Due to the small spherical nature of the sorbent materials, fluidization is a common design problem, and therefore, gas velocity through the sorbent beds is constrained. However, it is generally more difficult to regenerate the first sorbent alumina material than to regenerate the molecular sieve sorbents due to the fact water adheres to the alumina spheres with much greater force than carbon dioxide adheres to the molecular sieve sorbents.
In practice today, there are generally two types of adsorber systems. The first available system, a horizontal type adsorber, consists of two or more sequentially oriented sorbent beds in the flow path of a multicomponent gas stream. With horizontal adsorbers, feed gas flows through a first zone wherein certain contaminants are sorbed onto the sorbent material, and thereafter, the gas flows in the same or parallel direction to a second bed of sorbent material wherein other contaminants are removed. To regenerate horizontal adsorbers, the gas flow is reversed.
However, there are many inherent design problems with horizontal bed adsorbers. Limitations on diameter will dictate a maximum feed air or regeneration gas flow. Generally, both beds must be regenerated at the same time, and therefore, subjected to the same heat and mass flow. Horizontal bed adsorbers typically have no direct access to the lower bed of sorbent material, therefore, creating safety and degeneration concerns. To compensate, operators are forced to use more adsorbent material and endure the increase in pressure drop and energy usage during regeneration.
The second type of adsorber systems now available are termed radial bed adsorbers, wherein concentric beds of adsorbent material are displaced about a center axis. A gas flows perpendicular to the access through the one or more radial beds to adsorb one or more contaminants from the gas. Although radial bed adsorbers possess lower pressure drop and less tendency for fluidization of adsorbent versus horizontal bed adsorbers described above, there remain several significant disadvantages with radial bed designs. The bed dividers are subject to thermal stress problems and demand complicated support systems. A tendency for contaminated gas to by-pass one or more adsorbent beds, wherein adsorbents have settled even slightly, creates a severe operational problem. With radial designs, the vessel height is determined by one of the plurality of adsorbent beds. Because the bed thickness is determined by a minimum residence time of the contaminated gas in the adsorbent bed, a larger volume of other adsorbents than would otherwise be required must be used. This results in a higher cost than would otherwise be required for the second adsorbent material, and other associated costs with greater adsorbent beds than necessary.
As with the horizontal bed designs most commonly used, regeneration of either bed requires gas to be flowed through both beds concurrently. Therefore, the entire volume of regeneration gas must flow through both beds throughout the entire regeneration cycle. This results in an undesirable increase in energy usage, as well as an increased cycle time for regeneration.
Many attempts at improving the operation and performance features of both horizontal and radial bed adsorber systems have been made, meeting with various degrees of success. Due to the multitude of operational and design problems associated with adsorber systems presently available, an improved system to remove contaminants from a gaseous feed stream is much desired.